A heat treatment such as quenching is usually performed using a heat-treatment oil to provide a metal material with a desired hardness. The heat-treatment oil thus needs to exhibit an excellent cooling performance to enhance the hardness of the metal material. The heat-treatment oil is categorized into a cold oil designed to be used at a low oil temperature and a hot oil usable at a high oil temperature.
The cold oil usually contains a low-viscosity base oil and thus exhibits a high cooling performance with a high cooling speed. The cold oil, however, experiences a long-term vapor film stage, so that the usage of the cold oil may result in uneven quenching and, consequently, quenching distortion. Accordingly, the cold oil is frequently blended with a vapor-film rupturing agent to shorten the vapor film stage. Asphalt or polymer compound is generally usable as the vapor-film rupturing agent (see Patent Literature 1).